The present invention relates to a magnetic toner for use in developing latent electrostatic image formed in electrophotography, electrostatic recording, electrostatic printing, etc. More particularly, the present invention relates to a magnetic toner comprised of spheroidized particles.
The process of electrophotography generally comprises the following steps: providing a uniform electrostatic charge layer on the surface of a photoreceptor having a light-sensitive layer made of a photoconductive material; performing imagewise exposure to form a latent electrostatic image on the surface of the photoreceptor; developing the latent electrostatic image with a developer to form a toner image; transferring the toner image onto a receiving sheet such as paper; and fixing the transferred image either with heat or under pressure to form a copy image.
Dry developers used in the development step are generally classified as a two-component developer composed of a non-magnetic toner containing no magnetic material and a magnetic carrier, and a one-component developer formed of a magnetic toner containing a magnetic material. Of these two types, a one-component developer formed of a magnetic toner is preferred since toner density need not be adjusted and because the construction of a developing unit can be simplified.
Magnetic toner is usually transported to the developing area as it is carried on a sleeve by magnetism. If the magnetic toner particles have an irregular shape, the direction of their magnetization will not become uniform and difficulty is encountered in forming a developer layer of uniform density and thickness on the sleeve. The irregularly shaped toner particles have low fluidity and when supplied into the developing unit from above, they form a cap in the upper part of the developing unit creating a hollow inner portion. This phenomenon generally called "cavitation" renders the toner transport instable.
Further, magnetic toner particles that are irregularly shaped have many asperities on their surface and the area of frictional contact with the sleeve surface is insufficient to achieve rapid triboelectrification. This contributes to an increase in the proportion of weakly charged toner particles or those which have reverse polarity. As a result, fogging or fringing will occur in the toner image on the photoreceptor, and the reproduction of fine lines in the fixed image will be impaired. The term "fringing" as used herein means a phenomenon in which an unwanted toner which in chiefly composed of particles of reverse polarity is deposited in the non-image areas in the neighborhood of the latent electrostatic image on the photoreceptor. If fringing occurs, the consumption of toner particles that do more harm than good increases to render economical image formation difficult. Further, the image formed is incapable of faithful reproduction of fine lines. In addition, a substantial portion of toner particles of reverse polarity tends to remain on the photoreceptor without being transferred onto the receiving sheet and this increases the load on the cleaning device so greatly as to cause occasional insufficient cleaning.
In order to solve these problems, the triboelectrification of toner particles must be controlled and spheroidizing them is useful for this purpose. Various techniques have so far been proposed for producing spheroidized magnetic toner particles and they include the following:
(1) the surfaces of particles prepared by kneading, powdering and classifying steps are melted by hot air or other means using a spray dryer to obtain spheroidized particles (Unexamined Published Japanese Patent Application Nos. 56-52758 and 59-127662);
(2) the particles prepared by kneading, powdering and classifying steps are dispersed in a hot air stream and their surfaces are melted to obtain spheroidized particles (Unexamined Published Japanese Patent Application No. 58-134650);
(3) the particles prepared by kneading and coarse grinding are subjected to a fine grinding step while at the same time, the temperature of air flowing in is controlled to obtain spheroidized particles (Unexamined Published Japanese Patent Application No. 61-61627);
(4) granulation polymerization (Unexamined Published Japanese Patent Application No. 56-121048); and
(5) the particles prepared by kneading, powdering and classifying steps are effectively spheroidized by cyclic application of mechanical energy under impact force in a gas phase (Japanese Patent Application No. 62-68001).
In the first three methods, heat cannot be applied uniformly to all of the particles to be spheroidized and the melted particles will have an irregular shape or surface state. Further, the heated particles have a tendency to fuse together and the proportion of coarse particles will increase. As a result, the spheroidized particles will not be uniform in shape and size and in order to bring the distribution of their size into a desired one, another step of classification is necessary. Thus, it has been difficult to produce magnetic toners in high yield by methods (1) to (3). If unclassified particles having a broad size distribution are immediately used as toner, not only insufficiency or unevenness will occur in the density of the black solid area but also the image area composed of characters will suffer jumps, blocking of shadows, fogging and other problems.
In order to improve the efficiency of fixing with heated rollers, it is useful to incorporate waxes in magnetic toner particles. However, if particles containing waxes are thermally melted for spheroidization, the waxes will bleed on the molten surfaces by different degrees and it often occurs that the surface characteristics of individual particles have different levels in triboelectric series. Because of this nonuniformity in triboelectrification property, toner particles will not only be electrified in opposite polarity with respect to one another but they are also electrified weakly or in reverse polarity with respect to the sleeve. This causes instability in development and consequent fringing will increase the amount of toner particles that remain on the photoreceptor and which increase the load on the cleaning device to such a level that insufficient cleaning will often result. Further, concentrated fringes around fine lines will impair the reproducibility of character image.
The granulation polymerization technique adopted in the fourth method suffers the disadvantage of limited scope of applicable binder resins. Further, the toner production process takes a prolonged time and hence results in low yield.
The fifth method on which the present invention is an improvement has the following advantages:
i) in the absence of heating, toner particles will not fuse together during spheroidization;
ii) a wax will not bleed on the surface of toner particles;
iii) toner particles of reverse polarity will not be formed in large amounts; and
iv) short production time.
On the other hand, a major disadvantage of this method is that unduly fine particles and free magnetic particles will be generated because of crushing by mechanical energy.
If mechanical energy is applied to resin particles, they are not only spheroidized but also crushed into fine particles. With a one-component developer, small toner particles have a higher developing ability than large particles, so the fine particles produced will be a major factor that contributes to fogging in the initial period of use of a fresh developer. They also cause the toner particles to fly about in the developing device. Furthermore, if magnetic particles resulting from the crushing process are built up on the sleeve, insufficient toner transport will cause either uneven density or the formation of white streaks.
Developability or fogging can be controlled by adjusting the developing bias but it is by no means easy to broaden the range over which the developing bias can be adjusted since it requires a higher performance and hence costly device. Fine particles could be partly removed by performing classification after the spheroidization process but not all of them can be removed by this technique. On the contrary, the additional step of classification results in a lower yield and contributes to an increase in the production cost of developer through immediate increase in the running cost. If the impact energy applied for spheroidization is reduced to an extremely low level with a view to preventing the formation of fine particles, uniform and thorough spheroidization cannot be accomplished.